Convective Boundary Layer and Modeling of Dispersion.

Looping, bifurcation, and meandering of a plume are generally observed in a convective field. The blobby or puffy concentration patterns associated with these plumes are marked deviations from what is expected from either conventional K-theory or Gaussian distribution formulae. A numerical model has been developed for material dispersion in a convective boundary layer from both elevated and ground sources. Mechanistic formulation, rather than parameterization, or statistical behavior of planetary boundary layer (PBL) phenomena, has been used as a basis. The dispersion mechanism is considered to be due to mixing between the updraft and the downdraft. This model uses two universal constants, (turbulent entrainment constant, 'a', and decay constant 'A') and a mixing scheme directly supported by observations. We have examined the dispersion pattern from the elevated and ground sources. For elevated sources, the maximum concentration descends first to the ground level at some distance downwind, and then rises, depending on the inversion height, the mean wind and height at which material is released. The updrafts have a higher velocity than the downdrafts and consequently the downdrafts occupy a larger horizontal area. In some cases the updrafts and downdrafts are comparable and materials are caught equally in the updrafts and downdrafts. The concentration of material is split into two parts, one moving downward and the other upward. It is shown using the same mechanistic principles, that different convective situations cause different concentration patterns (for example, looping, bifurcating of a plume, and ascending of center line). The high shear against the surface modifies the mixing process at the bases of the plumes and this causes the materials to disperse more, both in the upwind and crosswind directions. In the case of strong shear or weak mean wind, the shear causes the material to stay more near the surface and source regions, and also enhances vertical mixing. We have simulated two observed cases; in one, material was released from an elevated source and for the other, material was released from the ground. The results of the model are in good agreement with the observations.